The present invention relates to a method of apportioning a quantity of material in the manufacture of molded products made from hardenable molding compounds. In such a method, a resin mat panel is cut into mats and the individual cut pieces are combined into mat packets and fed into the tool of a parallelism controlled press equipped with a path measuring system for measuring the distance between the clamping plates and the two tool halves, respectively, with the mat packets being compacted to become the molded products.
A programmable velocity and force controller for a press of this type is known, for example, from U.S. Pat. No. 4,076,780. The press disclosed in that patent includes four measuring sensors each attached at a corner of the press table supporting the lower mold half so as to determine the distance from the press table, and the parallelism of the press ram carrying the upper mold half and these measurements are used to control the press velocity and press force by means of a bracing system. For this purpose, the path measuring system composed of the four measuring sensors is connected with a control system which controls the hydraulic bracing elements at the four corners of the press table by way of servo controlled hydraulic valves to act against the press force introduced by way of the press ram. In this way, porous locations in the finished molded product caused by inaccurate guidance of the press ram can be avoided.
To be able to economically perform the processing of hardenable molding compounds (so-called SMC=sheet molding compounds) into molded products by means of pressing, it is desirable to substantially automate the system. This applies particularly for the processing steps between the resin mat as the preliminary material and the finished molded product. In the prior art processes, the foil-coated resin material supplied on a reel is unwound, the foil coating is removed and then the resin material is cut to size. Thereafter, the cut pieces are weighed and, grouped in packets or, more precisely, apportioned, before they are introduced into the press. After removal of the finished molded product from the pressing tool, the molded product must possibly still be deburred.
However, in the described processing steps, the consistency of the resin mats poses problems. The maximum or minimum weight per unit area of the resin mat often deviates greatly, sometimes considerably more than five percent, from the ideal average value. This may lead to considerable fluctuations in the thickness of the molded product and also to considerable burr formation on the molded product.
It is known to weigh the resin mat packet for a molded product and to compensate shortfalls in weight or excess weight by the addition or removal of material (corrections in the cut pieces). However, this does not consider specific weight tolerances of the individual cut resin mats stacked into mat packets and, because of the resulting differences in volume, this may lead to greater differences in thickness between one molded product and the next.
It is also known in the art to individually weigh the cut resin mat layers (cut pieces) and to correct the surface area of the resin mat layers to be cut thereafter, if there are discrepancies in weight. Although this makes it possible to keep deviations in weight at a minimum for the individual resin mat packets, it is impossible--in spite of the considerable expenditures for this method--to also consider the differences in volume in the cut resin mats. Moreover, other effects depending on the pressing process such as, for example, differences in burr formation on the finished molded products on the basis of differences in press forces also remain unconsidered.